Known image forming apparatuses operate to eject ink droplets while moving a carriage in a main scanning direction so as to form images on a recording medium moving in a sub-scanning direction.
In the image forming apparatuses having such a configuration, if the moving operation of the carriage is continued in a case where the clogging of a recording medium (recording sheet) i.e., “jamming” as a conveyance failure occurs, a heavy load is applied to the carriage and peripheral members thereof.
In view of this problem, a variety of techniques have been proposed with respect to image forming apparatuses that have a function of informing the user of the occurrence of the conveyance failure (jamming) and a method of eliminating an improper conveyance sheet so that the user is urged to eliminate a jammed sheet. However, the proposed techniques have a problem in detecting the jamming as early as possible to adequately stop the carriage.
A known jamming detection method determines the conveyance failure (jamming) if the output values of a carriage driving motor exceed a predetermined threshold in accordance with a characteristic that the output (current/voltage) values of the carriage driving motor for controlling speed fluctuate as the carriage comes in contact with foreign matter such as a recording medium (sheet).
Specifically, the jamming detection method has been disclosed in which the output (current/voltage) values of the driving motor at plural points when the carriage moves at a constant speed are compared with those of the driving motor at corresponding points when the carriage was previously driven in the same direction. Then, a determination is made of the occurrence of the conveyance failure if the difference between them exceeds a predetermined threshold (see, for example, Patent Document 1).
In this jamming detection method, however, it is necessary to set the threshold considering a margin about a fluctuation in motor output in accordance with variations in mechanism, ambient environment, time, etc. If the margin is too small, the fluctuation in the motor output at a normal operation where no jamming occurs is erroneously detected as the jamming. On the other hand, if the margin is too large, it takes time for the value of the motor output to exceed the threshold, resulting in a delay in detecting the jamming.
For this reason, in many cases, it is difficult to adequately set a margin during an actual operation, and there is a limit in accuracy for detecting the jamming for practical use.    Patent Document 1: JP-B2-2738802